Is 20 fabrics better than 10? Yes -- in a pattern designed around 20 fabrics. If you use only 10 fabrics in this pattern, you will have more repetition of prints and probably won't be happy with the result.
The more the better? I think this would apply to 'scrappy' quilts. Say I am making a pattern that calls for 4 different colors. I could use only 4 fabrics, or I could use 200 fabrics, 50 of each color. Will one quilt be better than the other? That is completely a matter of aesthetic taste. I can tell you that from across the room, the two quilts will look similar. It's up close that a large number of fabrics becomes a wonder to behold.
When I'm organizing my stash into projects, I collect the fabrics into a group and begin looking for a pattern that will work well for that group of fabrics. I would suggest you try this; if you collect a group of 10 fabrics that you love together, look for a pattern that calls for 10 fabrics. There are lots of free patterns on the various fabric manufacturer websites.
I agree that reading a few books on color/fabric selection for quilters would be helpful. Any book by the modern quilting duo Weeks Ringle and Bill Kerr will have good insights on color and fabric selection, and Jinny Beyer has her own unique take on color. Her technique is to settle on a color palette for the quilt, and then choose additional fabrics by blending from one color to the next.